1. Field of the Invention
This invention is in the field of alkali metal thermal to electric conversion (AMTEC), and in particular relates to the reduction of parasitic heat transfer within AMTEC devices.
2. Description of the Related Art
AMTEC devices consist of a high-pressure, high-temperature zone (900 to 1200 K) and a low-pressure, low-temperature (500 to 650 K) zone separated by a solid electrolyte structure that permits ions but not electrons of an alkali metal working fluid to migrate from the high to the low-pressure zone. Sodium is commonly used as the working fluid. xcex2 alumina normally functions as the separating electrolytic structure and is commonly referred to as the xcex2 alumina solid electrolyte (BASE). The BASE, a ceramic material, is an excellent sodium ion conductor and a poor electronic conductor.
The injection of thermal energy into the high-temperature zone and the rejection of thermal energy from the low-temperature zone create the pressure differential across the BASE. The pressure differential provides the energy needed to overcome the ionization energy of the sodium atoms, thereby creating positively charged sodium ions. The sodium ions readily pass through the BASE wall leaving an accumulation of electrons at the high-pressure interface. The negative and positive charge buildup at opposing interfaces creates an electrical potential across the BASE. This electrical potential can be used to drive an electrical load, i.e., the free electrons pass from the high pressure electrode (anode), through an electrical load, and back to the low pressure electrode (cathode) where they recombine with the sodium ions emerging from the BASE surface. The neutralized sodium atoms migrate in a vapor state through the low-pressure zone, condense on the cooled inner surface of the chamber, and return to the high-pressure zone via a capillary structure.
Inherent in the AMTEC cycle is the transfer of energy from the heat-input zone of the device to the heat-rejection zone through the evaporation, flow, and condensation of the alkali metal working fluid. This energy transfer through the exchange of latent heat is required for continuous operation of the AMTEC cycle. However, due to the presence of a temperature differential between the heat-input zone (high-temperature) and the heat-rejection zone (low-temperature), unwanted energy transfer occurs that is not inherent to the AMTEC energy conversion cycle. This unwanted, parasitic energy transfer reduces the thermal efficiency of the device. The energy transfer modes by which this parasitic energy transfer occurs include conductive and radiative heat transfer. While this parasitic heat transfer can not be completely eradicated, it can be minimized. A by-product of decreasing the parasitic heat transfer sometimes involves an increase in BASE temperature and sodium evaporation temperature. This is very fortuitous because increasing these parameters contributes to increased electrical power output.
Previous approaches to reducing this parasitic heat transfer have included positioning locally flat surface heat shields between the heat-input end (high-temperature) of the cell and the heat-rejection end (low-temperature) of the cell, reducing interior surface absorptivity and emissivity, and decreasing the cross sectional area of the enclosure wall that connects the high-temperature end to the low-temperature end. U.S. Pat. No. 5,929,371 employs a variety of cylindrical heat shield embodiments in the low-pressure zone that are intended to reduce the parasitic radiative heat transfer. These shields are effective in reducing the parasitic heat transfer, but they obstruct the sodium flow through the low-pressure cavity, which is a disadvantage. Obstruction of the sodium flow leads to a decrease in the pressure differential across the BASE, which reduces the power output of the cell. If properly designed, however, the reduction in parasitic heat transfer is sufficient to overcome the decreased power output resulting in a net increase in thermal conversion efficiency. The types of heat shields presented in the ""371 patent do not attempt to employ local directional control of thermal radiation nor do they directly claim to reduce the parasitic wall conduction. Independent analysis, however, has shown that the type of heat shields shown in FIG. 2 of the ""371 patent can actually cause an increase in the parasitic radiative heat transfer while decreasing the parasitic conductive heat transfer for an adiabatic external wall boundary.
It is the object of the present invention to reduce both the conductive and radiative parasitic heat transfer of AMTEC devices with cell modifications that minimally obstruct the flow of sodium from the high-temperature zone to the low-temperature zone, thereby increasing the thermal conversion efficiency.
The present invention consists of geometrically designed specular surfaces (grooves) within the low-pressure cavity of the AMTEC enclosure designed to reflect a significant amount of thermal radiation back to its source, the high-temperature BASE region. The result of this redirection of thermal radiation is a decrease in the parasitic heat transfer to the low-temperature end of the enclosure and an increase in the BASE temperature. Both of these effects cause an increase in thermal efficiency. In one exemplary embodiment of this invention, the geometrically designed surfaces are parallel, asymmetric wall grooves in a cylindrical AMTEC cell. Another embodiment involves an interior asymmetrically grooved cylindrical wall heat shield.